An optical component for handling electromagnetic radiation which is incident transversely to the plane is to be understood to mean herein a system of layers having reflecting or anti-reflecting properties with regard to electromagnetic radiation of a given wavelength.
A system of layers having reflecting properties and constructed as a multilayer thin-film component is known from U.K. Patent Application No. GB-A 2,020,842. The reflector described in said Application comprises a number (7-9) of dielectric layers having alternately a high and a low refractive index and thicknesses equal to 1/4.lambda., where .lambda. is the wavelength of the radiation to be reflected. The layers have been vapour-deposited and consist alternately of kryolite and zinc sulphide, or alternately of thorium fluoride and zinc sulphide. The disadvantage of this known reflector is that it exhibits physical defects which are inherent in layers formed by vapour-deposition. They locally have an insufficiently low absorption in the wavelength range of the radiation to be reflected and are insufficiently homogeneous and so have scattering or absorption centres As a result of this they are unfit notably for use in mirrors for high-power lasers.
It is the object of the invention to provide a multilayer optical component of the type mentioned in the opening paragraph which is well fitted for use as a mirror for high-power lasers.
According to the invention a multilayer optical component is characterized in that the substrate is a monocrystalline substrate having a lattice constant a.sub.o and that the stack of thin-film layers consists of a number of monocrystalline layers which have been grown epitaxially on the substrate and have a lattice constant which is substantially equal to a.sub.o.
A practical embodiment of the optical component in accordance with the invention is characterized in that the monocrystalline layers which, taken from the substrate, have an even number consist of the same material as the substrate. The phrase "taken from the substrate" as used herein means counting from the substrate with the substrate being numbered zero.
Within the scope of the invention the substrate may, for example, consist of monocrystalline gallium phosphide. Monocrystalline layers of alternately silicon and gallium phosphide may have been deposited thereon by means of hetero-epitaxy.
Alternatively, the substrate may consist of monocrystalline gallium arsenide. Monocrystalline layers of alternately silicon and gallium arsenide may have been deposited thereon by means of hetero-epitaxy.
According to a preferred form of the invention the substrate and the monocrystalline layers grown epitaxially on the substrate consist of a monocrystalline material having a garnet structure.
Epitaxial growth, for example from the liquid phase, of monocrystalline garnet layers physically speaking leads to substantially perfect layers as compared with vapour-deposited layers and which therefore have very few absorption centres and a minimum number of scattering centres. Various types of garnets can form combinations of layers having at least substantially equal lattice constants but different values of refractive index. Dependent on the difference in refractive index a smaller or a larger number of layers with alternately a high and a low refractive index may be used to realize a desired reflection.
A practical embodiment of the invention is characterized in that the substrate is of gadolinium gallium garnet (GGG). GGG can be obtained with a very high optical quality, i.e. a minimum of physical defects and negligible optical inhomogeneities.
For handling electromagnetic radiation in the infrared spectral range a further embodiment of the invention is characterized in that the monocrystalline garnet layers having an odd number when taken from the substrate consist of a material based on yttrium iron garnet and the layers having an even number when taken from the substrate consist of gadolinium gallium garnet.
For example, when alternate layers of yttrium iron garnet (YIG) (n=2.2) and GGG (n=2.0) are grown on a GGG substrate in thicknesses optimised for reflection, it is found that the reflection with an overall number of 2 layers is 18%, with an overall number of 10 layers is 49%, and with an overall number of 16 layers is 69%.
For handling electromagnetic radiation in the optical spectral range a further embodiment of the invention is characterized in that the monocrystalline garnet layers having an odd number when taken from the substrate consist of a material based on Y.sub.3 Al.sub.3 Sc.sub.2 O.sub.12 and the monocrystalline garnet layers having an even number when taken from the substrate consist of gadolinium gallium garnet.
Besides being useful as a reflecting or antireflecting element in lasers, the optical component according to the invention is also suitable for use in combination with an epitaxial magneto-optically active garnet layer. This may be present on the side of the stack of epitaxial monocrystalline garnet layers remote from the substrate or between the substrate and the stack of epitaxial monocrystalline garnet layers In these cases the thicknesses of the epitaxial monocrystalline garnet layers of the stack are chosen to be so that optimum reflection occurs for the wavelength of the electromagnetic radiation to be used.
The epitaxial magneto-optically active garnet layer, however, may also be provided between a first and a second sub-stack of epitaxial layers, the thicknesses of the layers of the first stack being chosen to be so that optimum reflection occurs for the wavelength of the electromagnetic radiation to be used, the thicknesses of the layers of the second sub-stack being chosen to be so that (optimum) anti-reflection occurs for the wavelengths of the radiation to be used. In this case the first substack may, for example, adjoin the substrate and the electromagnetic radiation to be treated may be incident via the second sub-stack.